Draggable Cat

CS 5010 Program Design Paradigms “Bootcamp”Lesson 3.3

© Mitchell Wand, 2012-2014This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

Introduction

In this lesson, you will learn how to make a Universe program that responds to mouse events, but more importantly, you will learn how to systematically add new features to a working program.

This is important, because we always build systems by starting with a small but working program, and then adding refinements and features.

Learning Objectives

• At the end of this lesson you should be able to:– design a Universe program that responds to

mouse events– list the steps in adding functionality to a working

program

draggable-cat: Requirements

• Like falling cat, but user can drag the cat with the mouse.

• button-down to select, drag to move, button-up to release.

• A selected cat doesn't fall. When unselected, cat resumes its previous pausedness– if it was falling, it will continue to fall when released– if it was paused, it will remain paused when released

Video: draggable-cat demo

YouTube link

in-cat? relies on Bounding Box

w = (image-width CAT-IMAGE)

h = (image-height CAT-IMAGE)

(x0,y0)

(x,y) is inside the rectangle iff (x0-w/2) <= x <= (x0 + w/2)and (y0-h/2) <= y <= (y0+h/2)

y = y0-h/2

y = y0+h/2

x = x0-w/2 x = x0+w/2

Information Analysis

• What are the possible behaviors of the cat?– as it falls?– as it is dragged?

• If we can answer these questions, we can determine what information needs to be represented for the cat.

• Let's write down the answers in graphical form.

Life Cycle of a falling cat

any other key event

unpaused paused

space bar

space bar

initially, cat is unpaused

any other key event

As the cat falls, it is either paused or unpaused. When the user hits the space bar, an unpaused cat becomes paused, and a paused cat becomes unpaused. Any other key event is ignored.

This is, of course, the same analysis that we did for falling-cat, but it's helpful to see it in graphical form.

Life Cycle of a dragged cat

drag: cat follows mouse

unselected selected

button-down in image

button-up

initially, cat is unselected

We can do a similar analysis for the cat as it is dragged. As the cat is dragged, it is either

selected or unselected. Here is a state diagram that shows what things cause the cat to change

from selected to unselected or vice versa.

Life cycle of a dragged, falling cat

(unselected, unpaused)

(selected, paused)

(unselected, paused)

(selected, unpaused)

initial state

drag: cat follows mouse

button downbutton up

space bar

space bar

button up button down

space bar

space bar

drag: cat follows mouse

tick: cat falls We can combine these to get a complete picture of how the cat

responds to various stimuli

Information Analysis: the Cat

• As before, our world consists of a single cat. • Since the cat can be dragged in the x direction,

we need to keep track of both the x position and y position of the cat.

• We also keep track of two Boolean values, telling us whether the cat is paused and whether the cat is selected.

• Here is the data definition, including the template.

Data Design for Cat(define-struct world (x-pos y-pos paused? selected?));; A World is a (make-world Integer Integer Boolean Boolean);; Interpretation: ;; x-pos, y-pos give the position of the cat. ;; paused? describes whether or not the cat is paused.;; selected? describes whether or not the cat is selected.

;; template:;; world-fn : World -> ??;(define (world-fn w); (... (world-x-pos w) (world-y-pos w) ; (world-paused? w) (world-selected? w)))

Life Cycle of Mouse Movements• What are the possible movements of a mouse?• Initially, the mouse enters the canvas (an "enter" event) and the button is

up. • While the button is up, the user can only do 3 things:

– move the mouse (a "move" event) or – move the mouse off the canvas (a "leave" event) or– depress the mouse button (a "button-down" event).

• While the button is down, again the user can do exactly 3 things:– move the mouse (this is called a "drag" event)– move the mouse off the canvas (a "leave" event) or– release the mouse button (a "button-up" event)

• When the mouse is off the canvas, no events are possible.• Similarly, we can draw a state-transition diagram for the movements of

the mouse.

mouse enters canvas

mouse leaves canvas

mouse is off-

canvas

button is up

button is down

button-down

drag mousebutton-up

move mouse

mouse enters canvasmouse enters canvas

Life Cycle of Mouse Movements

Mouse movements have their own life cycle. We've drawn the off-canvas events in a lighter color because most of the time we don't need to worry about them.

Information Analysis: Mouse Events

• Looking at the life cycle of a dragged cat, we see that only three mouse events are relevant: – "button-down" , – "drag", and – "button-up".

• Other mouse events, like "enter", "leave", or "move" are ignored.

• We can write a template for doing cases on MouseEvents for this application:

Template: Mouse Events; mev-fn : MouseEvent -> ??; STRATEGY: Cases on mev MouseEvent;(define (mev-fn mev); (cond; [(mouse=? mev "button-down") ...]; [(mouse=? mev "drag") ...]; [(mouse=? mev "button-up") ...]; [else ...]))

You don't have to write a template for this, but it's helpful, since you'll use the same set of cases several times in your program.

Getting your old program to work with the new data definitions

• Don't try adding the new features yet!• First, get all your old functions working with

the new data definitions.• Make sure your old tests work– Don't change your tests!– If you used mostly symbolic names for the test

inputs and outputs, so you should be able to just change those definitions.

– The tests themselves should work unchanged.

Testing your old functions(define unpaused-world-at-20 (make-world CAT-X-COORD 20 false false)) (define paused-world-at-20 (make-world CAT-X-COORD 20 true false))(define unpaused-world-at-28 (make-world CAT-X-COORD 28 false false)) (define paused-world-at-28 (make-world CAT-X-COORD 28 true false))...(check-equal? (world-after-key-event paused-world-at-20 pause-key-event) unpaused-world-at-20 "after pause key, a paused world should become unpaused")

same tests

adjusted values

Everything OK?

• Good. Now we are ready to move on to the new features.

Responding to Mouse Events(big-bang ... (on-mouse world-after-mouse-event))

world-after-mouse-event : World Integer Integer MouseEvent -> World

Look in the Help Desk for details about on-mouse

world-after-mouse-event;; world-after-mouse-event : ;; World Integer Integer FallingCatMouseEvent ;; -> World;; produces the world that should follow the given mouse event;; examples: See slide on life cycle of dragged cat;; strategy: cases on mouse events(define (world-after-mouse-event w mx my mev) (cond [(mouse=? mev "button-down") (world-after-button-down w mx my)] [(mouse=? mev "drag") (world-after-drag w mx my)] [(mouse=? mev "button-up") (world-after-button-up w mx my)] [else w]))

How to test this function?

• 3 mouse events (+ a test for the else clause)• cat selected or unselected– mouse works the same way whether the cat is paused or

not. • event inside cat or not.• 3 x 2 x 2 = 12 tests• plus test for else clause• plus: cat remains paused or unpaused across

selection.• Demo: draggable-cat.rkt

Draggable-cat readthrough

Remember, in the time since this video was recorded, we have revised our test framework and our treatment of KeyEvents and MouseEvents.YouTube link

The Iterative Design Recipe

• We started with a simple system, and we added some new features to it.

• In doing this, we were following a recipe.• We call this the “iterative design recipe”

because it tells us how to build a system by iteratively adding more complex features.

“iteratively” means “repeatedly” or “in stages”

The Iterative Design Recipe

Adding a New Feature to an Existing Program1. Perform information analysis for new feature2. Modify data definitions as needed3. Update existing functions to work with new data definitions4. Write wishlist of functions for new feature5. Design new functions following the Design Recipe6. Repeat for the next new feature

Summary

• In this lesson, you had the opportunity to– create a Universe program that responds to

mouse events– use the recipe for adding functionality to a

working program (the Iterative Design Recipe)

Next Steps

• If you have questions about this lesson, ask them on the Discussion Board

• Go on to the next lesson